The world is currently experiencing a severe environmental crisis due to high atmospheric carbon dioxide levels, in addition to an impending energy shortage. Both of these problems are a direct result from excessive fossil fuel utilization. The use of hydrogen (H2) as a fuel offers one highly attractive solution to these problems since the combustion of hydrogen (in a fuel cell, for example) produces only water and hence is free of emissions containing carbon and other pollutants.
Hydrogen gas contains more energy per mass unit than any known combustible fuel (approximately three times as much as gasoline). However, the use of hydrogen as a fuel has some drawbacks, since under ambient conditions it is an extremely low density gas. Hydrogen can be liquefied, cryogenically or with very high pressures (b.p., 1 atm.: −253° C.). These extreme conditions limit the potential widespread and large scale use of hydrogen.
A simple solution to these problems is to store the hydrogen chemically in a material that contains a very high weight percentage of hydrogen in a system that is capable of releasing the hydrogen on demand. Various compounds are known that contain a high gravimetric amount of hydrogen. Many of these compounds (e.g. lithium hydride (LiH, 12.8 wt % hydrogen) or lithium aluminum hydride (LiAlH4, 10.6 wt % hydrogen)) or alane (AlH3, 9 wt % hydrogen, Sandrock, G.; Reilly, J.; Graetz, J.; Wegrzyn, J. “Activated Aluminum Hydride Hydrogen Storage Compositions and Uses Thereof” U.S. Patent Application Publication No. 2007-0025908) suffer from being highly reactive or even potentially explosive and thus it is unlikely that commercial applications utilizing these compounds will be developed. Hydrogen generation by hydrolysis of magnesium hydride (MgH2) is a safer method but usage of metal hydrides will be problematic due to their inherent water sensitivity (U.S. Pat. No. 5,198,207, “Method for the Preparation of Active Magnesium Hydride-Magnesium Hydrogen Storage Systems, Which Reversibly Absorb Hydrogen”, Wilfried Knott, Klaus-Dieter Klein, Gotz Koerner, Th. Goldschmidt A G, Oct. 30, 1991) which can easily cause hydrogen formation under unwanted conditions.
The relatively benign compound ammonia borane (AB═NH3BH3, 19.6 wt % hydrogen) has been indicated by the United States Department of Energy to be number two on the list of potential hydrogen storage materials in terms of hydrogen content; the number one material is methane (CH4), combustion of which will lead to the obvious problem of further carbon dioxide emissions. Clearly, AB has significant advantages over all other known materials in applications where hydrogen is required as an energy carrier.
Goldberg and Heinekey (Denney, M. C.; Pons, V.; Hebden, T. J.; Heinekey, D. M.; Goldberg, K. I. J. Am. Chem. Soc. 2006, 128, 12048-12049) describe iridium complexes containing pincer ligands as catalysts in ammonia borane dehydrocoupling. The Baker group indicated that the dehydrocoupling reaction could be effected by a complex containing a base metal (nickel), although the activity of the system was only moderate (R. J. Keaton, J. M. Blacquiere, R. T. Baker J. Am. Chem. Soc. 2007, 129, 1844). A heterogenous, rhodium containing dehydrocoupling catalyst was reported by the Manners group where it was shown that the colloidal materials were capable of modest activity for dehydrocoupling (Jaska, C. A.; Clark, T. J.; Clennenberg, S. B.; Grozea, D.; Turak, A.; Lu, Z.-H.; Manners, I. J. Am. Chem. Soc. 2005, 127, 5116-5124). The dehydrocoupling of amine-boranes using metal complexes, where the metal is selected from manganese, iron, cobalt, nickel and copper has been reported (Blacquiere, J. M.; Keaton, R. J.; Baker, R. T. U.S. Patent Application Publ. No. US2007/0128475).